Past participle of 'swim'—used with 'have' to describe swimming that already happened (like 'I have swum across the lake').
From Old English 'swimman', a Germanic verb. 'Swum' is the strong/irregular past participle form that survives in modern English alongside the regular 'swim' forms.
English has about 200 irregular verbs left over from Old English—'swum' is part of this rebel group that refused to follow the normal '-ed' ending pattern, like 'swim-swam-swum' that's been around for over 1,000 years!
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